Packet Network Routing

1. A line card for use in a communications network node having a plurality of line cards, said line card, in use, being in communication with a routing processor shared by the line cards, said line card comprising: one or more line interfaces for connection to communication lines in a communications network, said interfaces being arranged to receive network packets addressed to nodes in said communications network; one or more interfaces arranged to transmit network packets processed by said line card to another of said line cards; said line card being arranged in operation to: i) receive a network state update via a communications line terminating at the line card; ii) pass said network state update to the shared routing processor for input into a routing process run by the shared routing processor which occasionally generates a set of routes for use by the line card; iii) receive said set of routes from said shared routing processor; and iv) route incoming packets on the basis of the received set of routes; said line card being arranged to further respond to receiving a network state update by calculating one or more interim routes taking said received network state update into account, and routing incoming packets over said interim routes until said set of routes is received from said shared routing processor.

2. A line card according to claim 1 wherein said line card is further arranged to calculate said one or more interim routes only on finding said network state update to be likely to impact the routing of packets arriving at the line card.

3. A line card according to claim 1 wherein said interim routing calculation calculates a route for one or more destination addresses found in one or more packets which arrive at said line card between the receipt of said network state update and the receipt of said set of routes.

4. A line card according to claim 1 further comprising bespoke hardware for performing said interim routing calculation.

5. A line card according to claim 4 wherein said bespoke hardware comprises a field programmable gate array programmed to perform said interim routing calculation.

6. A line card according to claim 4 wherein said bespoke hardware comprises an application-specific integrated circuit.

7. A router comprising a line card according to claim 1 .

8. A method of operating a line card for use in a router having a plurality of line cards, said line card, in use, being in communication with a routing processor shared by the line cards, said method comprising: operating said line card to: i) receive a network state update from another router via a communications line terminating at the line card; ii) pass said network state update to said routing processor for input into a routing process run by the shared routing processor which occasionally generates a set of routes for use by the line cards; iii) receive said set of routes from said shared routing processor; and iv) route incoming packets on the basis of the received set of routes; said method further comprising operating said line card to further respond to receiving a network state update by calculating one or more interim routes taking said received network state update into account, and routing incoming packets over said interim routes until said set of routes is received from said shared routing processor.

9. A non-transitory machine readable storage medium tangibly embodying a program of instructions executable by one or more processors on said line card to operate said line card in accordance with the method of claim 8 .

10. The method according to claim 8 further comprising: operating said line card to calculate said one or more interim routes only on finding said network state update to be likely to impact the routing of packets arriving at the line card.

11. The method according to claim 8 wherein said calculating one or more of the interim routes includes calculating a route for one or more destination addresses found in one or more packets which arrive at said line card between the receipt of said network state update and the receipt of said set of routes.